With the development of chip technology, in practical applications, the system is gradually required to provide a clock having a plurality of frequencies to support the system applications when there is merely one reference clock. Therefore, the phase locked loop (PLL) with an adjustable output clock frequency is required, and the voltage-controlled oscillator (VCO) is required to have a wider oscillation range and to operate stably.
FIG. 1 shows a conventional bias circuit, in which the VCO control voltage is converted into a current through a voltage-to-current converter, and then the current is input into the bias circuit and a delay unit. Once the current is input into the bias circuit, a feedback voltage Vfeedback is generated. The Vfeedback is applied to a non-inverting input terminal of an error amplifier. A reference voltage is input into an inverting input terminal of the error amplifier, and accordingly, an output voltage of the error amplifier is a temporary voltage V. The temporary voltage V controls the resistance of a voltage-controlled resistor (VCR). In this technical solution, the oscillation range of the VCO is relevant to the delay time, and the delay time is relevant to the amount of the current and the resistance of the VCR. Therefore, the circuit performs a delay control on the delay unit through the current and the resistance of the voltage-controlled resistor to realize the oscillation of the VCO.
However, in the prior art, due to the converter circuit for converting the control voltage Vc into the current I, when the Vc is lower than the threshold for turning on/off the voltage-to-current converter, the input tube cannot be turned on, and as a result, the entire circuit cannot work normally.
Meanwhile, as the delay unit requires the cooperation of the current and the VCR to perform the delay control and to realize the oscillation of the VCO, the Vc needs to be converted into I, and then converted into the temporary voltage V. As a result, during the process of converting the Vc into V, the grain errors are likely to be introduced, and thus affecting the accuracy of the resistance of the VCR, and influencing the stability of the VCO.